I don't know about a limitation on the length of a call, but it has never hung up on me. The quality just knocks me out. Even if the calling party has a $21 headset, like my best friend, the quality is DRAMATICALLY better than a phone line.
Today, I signed up for the unlimited monthly plan for $2.99 per month and started using it for my domestic calls when I'm sitting at my computer. Since Skype is almost true duplex, I have my RE20 for transmit and my computer speakers for receive. A USB headset is not even necessary. If the audio input is the Windows mix bus, I can play audio from the computer over the Skype call. Cool.